Data transmission by means of visible light is known from the prior art (“visible light communications”, VLC), which can be used, for example, as a supplement to conventional radio technology. Data can be transmitted in this situation, for example, by light-emitting diodes (LED). A data flow which is to be transmitted is in this situation transmitted, for example, in the form of modulations not perceptible to human beings.
Moreover, a more recent coding method for visible light is known, which is based on a color coding with elementary colors. Reference is made to such a method by persons skilled in the art by the term CSK (“Color Shift Keying”). Other older designations for this coding method are CCM (“Color Code Modulation”) or CMC (“Color Multiplex Coding”).
The functional principle of VLC making use of this color coding method includes, roughly speaking, of using an illumination mixed from several elementary colors for the additional transmission of data, wherein the individual elementary colors are modulated in such a rapid manner that, overall, the human eye recognizes one continuous mixed color. Usually the three elementary colors red, green, and blue are used for this, which can be emitted in a technically sophisticated manner by appropriate light-emitting diodes.
A detailed description of CSK can be found in the Modification Proposal to the Standard IEEE P802.15.7, Yokoi et al.: “Modified Text Clause 6.9.2.2”, Jan. 17, 2010, Document Identification “15-10-0036-00-0007”. One of the applications proposed for CSK is VLC, i.e. free-space communications with light.
In the modification proposal referred to, a receiver-end compensation is described for changes in the optical performance of the elementary color light-emitting diodes provided at the transmitter end. A compensation of the radiation emitted at the transmitter end is not provided for according to this modification proposal, due to the compensation at the receiver end only.
A method for transmitter-end compensation was proposed by the Applicant in an International Patent Application filed on 05.11.2010 with the Application Reference PCT/EP2010/066907 and the title “Method and arrangement for the stabilization of a color coding method for optical transmission of data”. In the method referred to, provision is made for a calibration message to be sent from the transmitter to the receiver. In the receiver, a piece of compensation information is ascertained by comparing a channel property derived from the received calibration message with a channel property stored beforehand in the receiver. This piece of compensation information is sent to the transmitter, in which, on the basis of the compensation information ascertained, an adjustment is made to at least one transmission parameter.
The proposed method does indeed make possible the transmitter-end compensation of the optical performance, but has the disadvantage that its implementation requires extensive modifications in the protocol of the control messages exchanged. For example, an additional control message is required for the transmission of the compensation information, which is not provided for in the protocol beforehand. A further disadvantage of the proposed method lies in the fact that storage of channel properties and a calculation of the compensation information on the basis of a stored channel property take place in the receiver, which imposes additional demands on the resources of the receiver.